<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f('加')">
<input type="button" value="减" onclick="f('减')">
<input type="button" value="乘" onclick="f('乘')">
<input type="button" value="除" onclick="f('除')">
<h3>
  结果:<span></span>
</h3>
<script>
  let n = document.querySelector("#i1");
  let m = document.querySelector("#i2");
  let s = document.querySelector("span");
  function f(x) {
    if(isNaN(n.value) || isNaN(m.value)) {
      s.innerText = "输入错误!"
      return;
    }
    switch (x) {
        case '加':
            s.innerText = parseFloat(n.value) + parseFloat(m.value);
            break;
        case '减':
            s.innerText = n.value - m.value;
            break;
        case '乘':
            s.innerText = n.value * m.value;
            break;
        case '除':
            s.innerText = n.value / m.value;
            break;
    }
  }

</script>
</body>
</html>